The present invention relates in general to a method and apparatus for transferring wafers (particularly silicon wafers) used in the fabrication of semiconductor devices and in particular to a method and apparatus for consecutively transferring wafers between carriers having a wide range of different spacings.
Electronic semiconductor devices, e.g., transistors and integrated circuits, are typically fabricated in large groups of wafers. The fabrication process can involve many steps, including heating, cleaning, and etching steps. Groups of wafers are transported between processing stages in carriers (also called boats). The carrier walls have a series of notches (or slots) to hold the wafers and space them apart by a certain distance (hereinafter referred to as the "spacing distance"). Different carriers can have different spacing distances. For example, a carrier designed to hold wafers while they are heated in a furnace could have notches spaced very close together (i.e., have a small spacing distance) to maximize the number of wafers in the furnace. On the other hand, a carrier intended to hold wafers during the cleaning and etching stages would normally have notches spaced further apart (i.e., have a large spacing distance) to facilitate manipulation of individual wafers. Different carriers may also hold different numbers of wafers.
The use of carriers with different spacing distances creates a problem when transferring wafers between carriers. Such transfers are typically accomplished by placing a first empty carrier on top of a second full carrier. The carriers are then flipped and wafers fall from the second carrier into the first carrier. If, for example, the first carrier has a spacing distance half as great as the second carrier, the flipping operation will empty the second carrier but only fill every other slot in the first carrier.
The problem is even more serious if the ratio of the two carriers' spacing distances is not an integer (e.g., 2/1) but rather a fraction (e.g., 2/3). If the spacing distance of the first carrier is two-thirds of the spacing distance of the second carrier, some of the slots in the first carrier will line up with slots in the second carrier, and the flipping operation will fill some of the slots in the first carrier. However, other slots in the second carrier will line up with the lands between slots in the first carrier. Thus some wafers will remain in the second carrier.
Some solutions have been proposed to the problem of transferring wafers between carriers having different spacing distances. See, for example, U.S. Pat. No. 3,934,733 and U.S. Pat. No. 3,949,891. However, the devices disclosed in these patents cannot be used effectively to transfer wafers between carriers if the ratio of the two carriers' spacing distances is not an integer.
The above patented systems also cannot effectively transfer one wafer at a time. In some situations, it may be desirable to only transfer a certain number of wafers and leave the rest of the wafers in the original carrier.